linux-kernel - [PATCH 3.16 127/157] x86/entry/64: Use JMP instead of JMPQ

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Message-ID: <lsq.1565469607.171840140@decadent.org.uk>
Date:   Sat, 10 Aug 2019 21:40:07 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, Denis Kirjanov <kda@...ux-powerpc.org>,
        "Josh Poimboeuf" <jpoimboe@...hat.com>,
        "Thomas Gleixner" <tglx@...utronix.de>
Subject: [PATCH 3.16 127/157] x86/entry/64: Use JMP instead of JMPQ

3.16.72-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Josh Poimboeuf <jpoimboe@...hat.com>

commit 64dbc122b20f75183d8822618c24f85144a5a94d upstream.

Somehow the swapgs mitigation entry code patch ended up with a JMPQ
instruction instead of JMP, where only the short jump is needed.  Some
assembler versions apparently fail to optimize JMPQ into a two-byte JMP
when possible, instead always using a 7-byte JMP with relocation.  For
some reason that makes the entry code explode with a #GP during boot.

Change it back to "JMP" as originally intended.

Fixes: 18ec54fdd6d1 ("x86/speculation: Prepare entry code for Spectre v1 swapgs mitigations")
Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
[bwh: Backported to 3.16: adjust filename, context]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 arch/x86/kernel/entry_64.S | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -267,7 +267,7 @@ ENDPROC(native_usergs_sysret64)
 	SWAPGS
 	FENCE_SWAPGS_USER_ENTRY
 	SWITCH_KERNEL_CR3
-	jmpq	2f
+	jmp	2f
 1:
 	FENCE_SWAPGS_KERNEL_ENTRY
 2: